Boilers for heating a cold fluid (e.g. water, steam, thermal oil or any other heating medium) with a hot fluid (e.g. hot gases) with tubes are well known. Many improvements were provided in the past. In order to enhance the efficiency, number of isolated passages was increased by adding separators, plates or baffles among the tubes. These additional parts among the tubes are exposed to the hot fluid and thus require maintenance and decrease the availability of the boiler. These additional parts could also generate noise nuisance.
In order to enhance the efficiency, economizers are provided to be installed outside the boiler for saving energy released in the hot fluid escaping from the boiler. This type of economizer is separated to the boiler and need an external assistance (e.g. pump) for the circulation of the cold fluid trough the economizer. The external assistance consumes energy and thus decreases the global efficiency of the boiler.
Also, the transfer of the hot fluid from one passage to another is done with a particular pattern of tubes at the ends of the passages. That increases the number of types of tube to keep in inventory at the different level of the supply chain.